Thixotropy in conventional paint and coating compositions is desired because thixotropy helps to reduce pigment separation and settlement. In addition, thixotropy improves the transfer of the paint to the brush or roller and reduces the tendency of the paint to drip and splatter as it is being applied. Thixotropic recovery also enables a thicker film of paint to be applied without sagging.
Thixotropes are generally divided into two categories: additive thixotropes and thixotropic resins. Additive thixotropes include fumed silicas, sulphonated castor oil, organo-modified clays, hydrogenated castor oil based additives and organic wax pastes. These thixotropes are typically added to the paint composition during the pigment dispersion phase of paint manufacture. Thixotropic resins can be prepared by reacting an oil soluble dimer acid based polyamide resin onto an alkyd resin using a carefully controlled amide/ester interchange reaction. Thixotropy develops from the hydrogen bonding between the --NH groups and the adjacent --OH groups. The synthesis of thixotropic alkyd resins is typically carried out at high temperature and between amine and alkyd resins that have been stripped of solvents.
It has been found that thixotropic resins may be prepared at low temperature from solvent borne alkyd resin and solvent borne polyamide resin. Furthermore, thixotropic resins can easily be prepared from more than one alkyd resin since there is no need to strip the organic solvent from the alkyd resin prior to reaction with the polyamide resin. Paints and coatings prepared from these thixotropic resins exhibit superior flow leveling and sag resistance. Conventional thixotropic additives may also be incorporated into the paint and coating compositions to further enhance thixotropy.